We live in a dynamic world, and the warming climate is driving changes in ocean chemistry. Some of the most prominent changes include warming surface water temperatures, increased carbon dioxide concentrations that lower ocean water pH and cause ocean acidification, and increased atmospheric dust deposition that can release nutrients and trace metals into the ocean. These drivers can have important repercussions on not just ocean chemistry, but marine primary production.

One of the major sources of essential nutrients to the ocean is mineral dust, which can deposit nitrogen, phosphorus, and iron, a biologically-important trace metal, into the open ocean, making mineral dust deposition an important factor controlling phytoplankton growth. The Saharan and Gobi Deserts are major sources of mineral dusts, which can be transported from source areas over thousands of kilometers. Climate change models forecast increased levels of dust deposition on the open ocean, and so the study here set out to understand how dust additions impact diatom growth.

An example of the diversity of diatoms. Diatoms are cool because they build their outer shells with silica, and so some have a glassy appearance. Photo credit: Shelly Wu.

Diatoms are a type of phytoplankton responsible for around 20% of global primary production, which makes them important in driving nutrient cycling in surface waters. Diatoms are dominant species in the natural phytoplankton assemblages, especially in nutrient-rich ocean regions, such as continental margins and upwelling areas. Marine phytoplankton growth is controlled by the availability of macronutrients (Nitrogen, Phosphorus, Silica) and a number of trace metals, such as iron.

The Study

The main objective of the present study was to evaluate how global change processes, particularly dust deposition, sea-warming, and ocean acidification, could affect diatom growth and nutrient biogeochemical cycles. To achieve these objectives, the researchers conducted laboratory experiments on a common marine diatom, Chaetoceros socialis, under controlled laboratory conditions of temperature, light intensity, carbon dioxide concentrations, and mineral dust input.

The authors wanted to examine how diatom growth was affected under several combinations of current and near-future climate change-induced conditions of temperature, carbon dioxide concentrations (pCO2 for short), and dust deposition. In order to test all possible combinations of these variables, four pCO2/temperature treatments were examined:

Present-day treatment: 13˚C and 400μatm pCO2;

High temperature (ocean warming) treatment: 18˚C and 400μatm pCO2, with only the increase of temperature;

High pCO2 (ocean acidification) treatment: 13˚C and 800μatm pCO2, with only the increase of pCO2; and

Greenhouse treatment: 18˚C and 800μatm pCO2, with simultaneous increases of temperature and pCO2.

To add mineral dust to the experimental treatments, the researchers collected dust from the Kubuqi Desert, which is part of the Gobi Desert in China. The amount of dust that they added in their experiment represented a high dust loading, which is approximately 50 times higher than dry dust deposition.

Results

Effects of ocean warming and acidification

Diatom growth rate and abundance was highest in the present-day treatment (400 μatm and 13˚C), while the lowest growth rate was observed in the Greenhouse treatment (800μatm pCO2 and 18˚C, see figure below). Overall, increasing carbon dioxide concentrations had a negative effect on diatom growth rate. In addition, this study reported lower cell abundances and growth rates at 18 C compared to 13 C for both 400 μatm and 800 CO2 conditions.

The authors reported only the effects of dust deposition from the high temperature treatment (400 μatm pCO2 + 18˚C). In the treatment without dust addition, diatom abundance increased initially, but soon after growth reached a maximum, growth began to decrease. In the dust addition treatment, diatom growth rates and cell abundance increased significantly compared to the no-dust treatment. These results indicate that dust addition and the release of dust-bound nutrients and trace metals stimulated diatom growth.

Modified Figure 2 from Li et al. 2017 via Creative Commons License. Here, diatom cell count initially increases with or without dust addition, but with dust, the diatoms grow for longer compared to those without dust.

Primary production likely will change with climate-induced drivers

This study focused on one species of diatom and its response to climate change factors. It is important to remember that while climate change may lead to a decline in marine phytoplankton biomass, the results reported here may not hold for other diatom species. However, the results from this study clearly show that the diatom Chaetoceros socialis responded negatively to warmer water temperatures and higher carbon dioxide concentrations. But, even at high temperatures, the diatoms in this study grew better with the addition of mineral dust, and this was likely the result of important trace metals, such as iron, and macronutrients, including nitrogen and phosphorus, dissolving in the water, rendering them available to diatoms.

(Featured image) A close-up of a diatom, an incredibly important primary producer in marine food webs. Photo credit: Shelly Wu.

An important take-away from this study is that seawater pH and temperature, the two most important variables in all chemical and biological processes, could alter the availability of trace metals to phytoplankton. In this experiment, the researchers saw a doubling of dissolved iron concentrations in the high CO2 treatment compared to the low CO2 treatment; when more CO2 is dissolved in seawater, the pH drops, and lower pH shifts the balance between different forms of iron. In the case of the high CO2 treatment, the lower pH likely resulted in an increase in the fraction of dissolved iron, which made more iron available for diatom growth.

From this study, it seems that dust is beneficial for diatom growth, but is this the case for other diatoms and other types of phytoplankton? More studies on the effects of dust deposition, ocean warming, and acidification on marine phytoplankton will be needed to improve our understanding on how multiple climate drivers influence marine primary productivity.

Kate is a 2nd year PhD student in the Biological Sciences Department at the University of Notre Dame, and holds a Masters in Environmental Science & Biology from SUNY Brockport. She studies benthic-pelagic ecosystem linkages in the Great Salt Lake, Utah. Outside of lab and field work, she enjoys running and kickboxing.